Titanium dioxide has been known to be produced by drying or calcining of Titanium hydroxide. There are various methods of producing Titanium hydroxide from different types of Titanium compounds.
Three crystalline forms of Titanium dioxide are known in the art: Anatase, Rutile and Brookite, Anatase or amorphus Titanium dioxide partially crystallized into Anatase, are commonly used as catalysts (for example see U.S. Pat. Nos. 4,388,288 and 4,422,958).
In most catalytic processes carried out in gas phase or liquid phase it is extremely viable to use catalysts with developed meso and macropore structure, which accounts for a high catalytic activity and for a high rate of reagents diffusion inside granules or shaped blocks of catalysts, and diffusion of reaction products outside the catalysts structure.
Mesopore structure is determined by the method of the active material precipitation from a solution and its subsequent treatment.
Macropore structure is determined mainly by the process of granulation or shaping.
Many catalytic processes are carried out under increased temperatures where Titanium dioxide undergoes shrinkage. Its active surface area is reduced and its activity is also decreased. Since the heat treatment will take a long period of time, the Titanium dioxide aging process leads to its structure transformation from Anatase to rutile crystalline form, and these alterations cause a significant decrease in its catalytic activity. Since Titanium dioxide is quite an expensive material it is most desirable that such catalyst should possess a prolonged effective period of use.
In order to increase the thermal stability of Titanium dioxide, various additives were suggested. Thus, in order to increase its stability, when it is obtained from Titanium oxychloride by precipitation with an aqueous solution of ammonia at a pH of 7.8, at least one stabilizing agent is incorporated. Useful agents for this purpose may be selected from aluminium, sodium, potassium, calcium or other chlorides, nitrates and powdery silica. The criterion of thermal stability is based on the change in the specific surface area of a sample calcined at 575 degrees C. for seven hours, compared with a sample calcined only for one hour at the same temperature (see French patent application No. 2,621,577 and European Patent Application No. 0311,515).
Several investigations were performed in the field of Titania/Silica mixed oxides preparation. The idea of this approach is a coprecipitation of titanium hydroxide and silicic acid,
Titania/Silica mixed oxides preparation and their use as supports for Nickel catalyst were described (Journal of Catalysis 105, p. 511-520 1987). As mentioned therein, a mixed oxide was precipitated from a mixture of TiCl4 and SiCl4.
Precipitation of titanyl sulfate in the presence of a powdery dry silica (SYLOID-74) was carried out in order to prepare samples containing 20%, 40% and 80% by weight TiO2 and investigations with these precipitates as catalyst for selective catalytic reduction of NOx, were described in Applied Catalysis A, General 139, 1996, pages 175-187.
The method for the preparation of Titania-Silica mixed oxides with alkoxide sol-gel method was described in Journal of Catalysis, 153, p. 165-176 (1995). As mentioned therein, tetra-isopropoxy-titanium and tetra-methoxysilicon were used as starting materials. Various titania-silica aerogels obtained by an alkoxide-sol-gel route, were prepared and tested in the reaction of epoxidation of olefins (Journal of Catalysis 153, 177-189, 1995),
Crystalline titanium silicates having specific adsorption and catalytic properties were described in literature (Advances in Catalysis, Vol. 41, 253-327, 1996).
The above brief review illustrates the importance attributed to the subject of obtaining a novel type of titanium dioxide.
It is an object of the present invention to provide a novel modified Titanium dioxide and methods for its preparation. It is another object of the present invention to describe various uses of the novel modified Titanium dioxide.